An ionic polymerization method linear low-density polyethylene (LLDPE) as a copolymer of ethylene and an .alpha.-olefin has widely been used as a film, a sheet, a hollow container and a raw material for injection molding because of its excellent mechanical strength such as impact strength, tensile strength, and environmental stress-crack resistance, etc. However, when the above LLDPE is produced by using a solid catalyst, there arises a problem of tackiness of the molded article and it was not necessarily satisfactory. To solve the problem, LLDPE using a vanadium catalyst has been studied. However, there arises a problem of coloring due to a large amount of the catalyst residue in the polymer and it was not necessarily satisfactory. To solve those problems, various metallocene catalysts have recently been developed as a homogeneous catalyst.
As a method for industrially producing an olefin polymer at high temperature using a catalyst for olefin polymer, the following two methods are practiced at present. The first method is a method for polymerizing an olefin under the condition where a polymer is molten using a solvent such as cyclohexane, etc., which is generally referred to as a "high-temperature solution method", for example, a method for polymerizing an olefin under the condition of 120 to 250.degree. C. and 5 to 50 kg/cm.sup.2. The second method is a method for conducting polymerization in a state where the formed olefin polymer is molten in an olefin in a super critical fluid state at high temperature under high pressure in the absence of a solvent, which is generally referred to as a "high-pressure ionic polymerization method". These high-temperature solution polymerization method and high-pressure ionic polymerization method have advantages that a reactor is compact and replacement of a monomer can be easily performed.
For example, a method for obtaining an olefin polymer by using a metallocene catalyst under high pressure is disclosed in Japanese Patent Publication (Kohyo) No. Hei 1-503788, Japanese Patent Publication (Kokai) No. Hei 7-157508 and U.S. Pat. No. 5,408,017.
However, Japanese Patent Publication (Kohyo) No. Hei 1-503788 discloses a method for producing an olefin polymer by using a catalyst containing bis(n-butyl)cyclopentadienyl zirconium dichloride and methylaluminoxane in a high-pressure ionic polymerization method. Although the polymerization pressure is considerably high such as 1000 bar or more, the molecular weight of the resulting ethylene-.alpha.-olefin copolymer was insufficient. This method has an advantage such that an olefin polymer having a narrow molecular weight distribution and a composition distribution can be efficiently obtained and a film produced by using the olefin polymer has no feeling of tackiness, but the transparency of the film produced by using the resulting polymer is unsatisfactory.
Furthermore, Japanese Patent Publication (Kokai) No. Hei 7-157508 discloses a method for producing an olefin polymer with a catalyst obtained by using diphenylmethylene(cyclopentadienyl)(fluorenyl)zirconium dichloride, triisobutylaluminum and N,N-dimethylaniliniumtetrakis(pentafluorophenyl)borate in a high-pressure ionic polymerization method, and U.S. Pat. No. 5,408,017 discloses a method for producing an olefin polymer with a catalyst obtained by using dimethylsilylbis(4,5,6,7-tetrahydro-indenyl)zirconium dimethyl and N,N-dimethylaniliniumtetrakis (pentafluorophenyl)borate in a high-pressure ionic polymerization method. In these polymerization methods, although the polymerization pressure is high as not less than 900 kg/cm.sup.2 and 1300 bar, respectively, the molecular weight of the ethylene-.alpha.-olefin copolymer obtained by any method was insufficient and the transparency of the film produced by using these polymers was unsatisfactory in the same manner as described above.
Japanese Patent No. 2571280 discloses an ethylene copolymer having a small Mw/Mn, narrow molecular weight, large MFR.sub.10 /MFR.sub.2 and excellent fluidity, but the transparency and tear strength of the film produced by using this ethylene copolymer were not necessarily satisfactory.
Japanese Patent Publication (Kohyo) No. Hei 7-500622 discloses an olefin polymer wherein the melt flow ratio, Mw/Mn, shear stress at which a gross melt fracture arises, and number of melting points are specified, but the transparency and tear strength of the film produced by using this polymer were not necessarily satisfactory.
It is one of features that an olefin polymer having narrow molecular weight distribution and composition distribution is obtained by using a metallocene complex as a catalyst component, but there is a problem that control of density and polymerization activity, particularly a polymerization activity at the reaction temperature which is efficient for industrial production, are insufficient.